1. Field of the Invention
This invention relates generally to a blood flow measurement system, and more particularly to a blood flow measurement system for obtaining blood flow rate, blood flow velocity and other blood flow information based on the intensity of scattered light received from in vivo tissue irradiated with coherent light.
2. Description of the Prior Art
Apparatuses used for obtaining blood flow data include electromagnetic stromuhrs for use with a single blood vessel and laser stromuhrs for noninvasive optical measurement of blood flow at the skin surface.
In the electromagnetic stromuhr, a measurement probe including a magnetic pole for producing a magnetic field and an electrode for detecting electromotive force is attached to the blood vessel so as to enclose it and the mean flow velocity of erythrocytes flowing through the probe is measured. Based on the assumption that the inside diameter of the measurement probe attached to the blood vessel is equal to the diameter of the blood vessel, the sectional area of the blood vessel is defined in terms of the sectional area of the measurement probe.
The blood flow rate is therefore calculated from the product of the average flow velocity and the sectional area of the probe. A problem tends to arise when the measurement is conducted at multiple points on a plurality of closely spaced blood vessels, however, because electromagnetic induction occurring between proximate measurement probes may interfere with the measurement.
On the other hand, the laser stromuhr enables noninvasive measurement of changes in blood flow in the skin and outputs a value corresponding to the blood flow. The laser stromuhr receives as its signal the light scattered from large numbers of erythrocytes flowing through the in vivo tissue. By frequency analyzing the light quantity as it varies with time owing to the mutual interference among the scattered light rays from the large number of erythrocytes, it extracts a quantity dependent on the macroscopic flow velocity of the large number of erythrocytes and a quantity corresponding to the number of erythrocytes in the irradiated region and calculates the blood flow value.
The prior art technologies are characterized by the methods they use for frequency analysis of the light quantity variation. For example, in one of these methods the value F corresponding to the blood flow rate is defined as ##EQU1## where .omega. is the angular frequency and P(.omega.) is the power spectral density. (See: Nilsson, G. E., Tenland, T. and Oberg, P. A.: Evaluation of a laser Doppler flow meter for measurement of tissue blood flow. IME-27, 597-604, 1980) In another method described in Japanese Patent Laid-open Publication No. Sho 60(1985)-203235, for example, the power spectrum obtained by frequency analysis is plotted in a full logarithmic coordinate system and the slope is defined as the value corresponding to the skin blood flow.
When the performances of the prior art apparatuses were compared by measuring the flow of light-scattering particles as a simulated blood flow, it was found that the coefficient between the flow velocity and the output value from the apparatus is different due to the difference in adopted methods. (See: Okada, H., Fukuoka, Y, Minamidani, H., Sekizuka, E., Ohtzuka, T. and Bokuzawa, S: Problems concerning quantitativeness of laser system stromuhr. Papers to be read at the Fourth Japanese ME Association Autumn Conference, Medical Electronics and Bioengineering, Vol. 27, Special Autumn Edition (1989))
Moreover, actual measurements have to be made in the presence of movement and vibration caused by the respiration, pulsation etc. of the living tissue that is the measurement subject. Different systems treat these external disturbances differently and the electrical low-pass filters they use for averaging over time have different characteristics. Time response characteristics are often sacrificed in order to achieve stable measurement.
Japanese Patent Laid-open Publication Nos. Hei 4(1992)-193158 and 4(1992)-193159 teach multiple point measurement methods. Specifically, japanese Patent Laid-open Publication No. Hei 4(1992)-193159 teaches an apparatus and method for deriving two dimensional blood flow information by scanning a laser beam intermittently so as to repeatedly move and stop the beam spatially and obtaining scattered light information relating to the blood flow in the in vivo tissue when the beam is stopped. In order to measure a large number of points within a two-dimensional plane as quickly as possible by this method, it is necessary to suppress the effect of vibration on the measurement at the measurement points while also conducting the measurement with good time response characteristics. The maintenance of the time response characteristics is of importance also in the measurement by points.
The comparison between the electromagnetic stromuhr and the laser stromuhr shows that, owing to its measurement principle, the laser stromuhr is free of the problem of magnetic inductance between measurement probes experienced by the electromagnetic stromuhr discussed earlier and, as such, has the advantage of being able to bring the measurement points close together for measuring the blood flow rate in a plurality of closely spaced blood vessels. If, therefore, a stromuhr operating on the principle of measuring scattered laser light could be used in place of the probes of an electromagnetic stromuhr, the problem of magnetic induction between measurement probes would no longer have to be taken into account.
Use of a laser stromuhr in place of an electromagnetic stromuhr for directly displaying blood flow rate is in fact difficult, however, because current laser stromuhrs produce measurement values that change with the individual measurement conditions and are able to display only a relative value or the amount of change.
The inventor's approach to improving the poor time response of the prior art laser stromuhr and overcoming its drawback of being able to display only relative values was to focus on improving the method of analyzing the time-course variation components of the scattered light quantity.
For improving the accuracy of signal analysis it is necessary to extract the substantial characteristics of the power spectrum without interference from noise. Research has been directed to signal processing methods that presume a peak frequency, a maximum frequency or the like within a specific frequency band, however, in large part because it has been held possible to obtain a Doppler signal with respect to the scattered light signal generation mechanism.
In fact, however, the practically obtainable electric signal does not include the peak frequency peculiar to a Doppler signal and is observed as a dynamic speckle signal caused by the mutual interference between scattered light beams from the moving light-scattering particles. It is therefore appropriate to provide a method of the signal processing based on the concerning signal characteristics and it is effective to provide an analysis method that takes the characteristics of the power spectrum function into account.
The conventional method of removing noise has been to measure the power spectrum of the dark noise and to remove the measured component from the power spectrum at the time of measuring the blood flow, thus calculating the power spectrum for a subject under measurement. Since the noise component is also generated at the time of measurement, however, it is preferable from the viewpoint of signal processing to estimate the noise component from the measured power spectrum and then remove it.
On the other hand, a method which attempts to remove the noise component by taking the average of the power spectrum over time has also been adopted. Since this involves the integration of a signal that varies with time, however, it sacrifices the time response characteristics and, while stable, requires sampling over long periods during which the measurement subject has to be restrained. In some cases, therefore, there is a restriction on the type of subject.